The present application relates generally to the use of liquid desiccants to dehumidify and cool, or heat and humidify an air stream entering a space. More specifically, the application relates to an optimized system configuration to retrofit 2- or 3-way liquid desiccant mass and heat exchangers that employ micro-porous membranes to separate the liquid desiccant from an air stream in large commercial and industrial buildings while at the same time modifying existing Heating Ventilation and Air Conditioning (HVAC) equipment to achieve a significant reduction in electricity consumption in the building.
Desiccant dehumidification systems—both liquid and solid desiccants—have been used parallel to conventional vapor compression HVAC equipment to help reduce humidity in spaces, particularly in spaces that require large amounts of outdoor air or that have large humidity loads inside the building space itself. (ASHRAE 2012 Handbook of HVAC Systems and Equipment, Chapter 24, p. 24.10). Humid climates, such as for example Miami, Fla. require a lot of energy to properly treat (dehumidify and cool) the fresh air that is required for a space's occupant comfort. Conventional vapor compression systems have only a limited ability to dehumidify and tend to overcool the air, oftentimes requiring energy intensive reheat systems, which significantly increase the overall energy costs, because reheat adds an additional heat-load to the cooling coil. Desiccant dehumidification systems—both solid and liquid—have been used for many years and are generally quite efficient at removing moisture from the air stream. However, liquid desiccant systems generally use concentrated salt solutions such as ionic solutions of LiCl, LiBr or CaCl2 and water. Such brines are strongly corrosive, even in small quantities, so numerous attempts have been made over the years to prevent desiccant carry-over to the air stream that is to be treated. In recent years efforts have begun to eliminate the risk of desiccant carry-over by employing micro-porous membranes to contain the desiccant.
Liquid desiccant systems generally have two separate functions. The conditioning side of the system provides conditioning of air to the required conditions, which are typically set using thermostats or humidistats. The regeneration side of the system provides a reconditioning function of the liquid desiccant so that it can be re-used on the conditioning side. Liquid desiccant is typically pumped between the two sides. A control system is used to properly balance the liquid desiccant between the two sides as conditions necessitate and that excess heat and moisture are properly dealt with without leading to over-concentrating or under-concentrating the desiccant.
In large stores, supermarkets, commercial and industrial buildings, energy is wasted because the existing unitary HVAC units serving the building do not adequately dehumidify the ventilation air that they provide to the building. This excess humidity winds up being condensed out of the air with excess energy usage from refrigeration and freezer equipment inside the building, which creates a load on that equipment resulting in a higher than necessary energy consumption.
Older buildings typically have been designed with HVAC equipment that recirculates a large portion (80-90%) of the air from the space through its cooling coil. The equipment takes in approximately 10-20% of fresh outside air, which as discussed above requires dehumidification, which is not adequately done by this equipment. At time of construction and design, engineers will sometime add a desiccant system to create the necessary dehumidification, but such equipment is heavy, complex and expensive and is not retrofitable on buildings that were not originally designed to accommodate them.
There thus remains a need to provide a retrofitable cooling system for buildings with high humidity loads, wherein the dehumidification of outside air can be accommodated at low capital and energy costs.